Hi Marmalady (and HTC, of course) --
I will happily bow to your experience if you've made that icing and it withstood heat without the guests landing in the hospital later ;-). My caution was intended very generally in any event although I still stand that one must err on the cautious side when preparing food for hot weather eating (I've had salmonella in my system -- six months of misery and I tend to look at anything cross-eyed since then to see if just shouts "SALMONELLA!". I'm sure this is a case of "once burned, twice shy"!)
As for the cream cheese idea, I still don't recommend it, for melting at least. I'm writing from a place with pretty hot summers (Greece) and it sure doesn't hold up here. Of course proportions matter greatly, but I still wouldn't vote that as No. 1.
And I hold to my claim that any amount of grease will interfere with royal icing. Feel free to try, but I've regretted it.
Lulu, poured fondant is different from rolled fondant. It's georgous, but it's a different animal. HTC, don't misunderstand me: I agree with you 100% about the taste of fondant although home-made is pretty inoffensive (some people actually like fondant), however, I still think it's unbeatable for keeping something fresh, especially in heat. And again, for decorating on, it's a beautiful 'blank canvas'.
The way I've come to think about it (because I really do have to use it) is more as a 'box' for the cake ... or a 'wrapping' -- i.e., something you strip off after you admire it, and toss it to the side. That helps ME deal with the quandry of serving a cake you've gone out of your way to make delicious covered in something you find bordering on repulsive...
Anyways, that's neither here nor there since you probably won't use fondant and will go with the (very good indeed) suggestion of icing at picnic. Either way, good luck and happy picnicing!